1. Field of the Invention
The present invention relates to a device for converting a complex-valued bandpass signal into a digital baseband signal. The invention further relates to a transmitting receiving device and an integrated circuit having such a device.
2. Description of the Background Art
The invention is in the field of telecommunications. In particular, it is in the field of digital telecommunications systems, in which a plurality of transmitting/receiving devices access a certain frequency band or a part thereof according to a multiple access method. In this context, it is necessary in the receiving unit of each transmitting/receiving device to convert a bandpass signal, for example a radio signal received through an antenna or an intermediate-frequency signal derived therefrom, into a digital baseband signal before the data values contained therein, which originate from another transmitting/receiving device, can be detected.
Although they are applicable in principle to any desired wireless or wired telecommunications systems, the present invention and the problem on which it is based are described below using a “ZigBee” communications system per IEEE 802.15.4.
Wireless Personal Area Networks (WPANs) may be used for wireless transmission of information over relatively short distances (approximately 10 m). In contrast to Wireless Local Area Networks (WLANs), WPANs require little, or even no, infrastructure for data transmission, so that small, simple, energy-efficient and low-cost devices can be implemented for a wide range of applications.
IEEE Standard 802.15.4-2003 (herein after referred to as 802.15.4) specifies low-rate WPANs which, with raw data rates of up to 250 kbit/s and fixed-position or mobile devices, are suitable for applications in industrial monitoring and control, in sensor networks, in automation, as well as in the field of computer peripherals and for interactive games. The ability to implement the devices very simply and economically is critical for such applications, as is an extremely low power requirement. Hence, this standard aims for battery lifetimes of several months to several years.
At the level of the physical layer, IEEE Standard 802.15.4 specifies a total of 16 (carrier frequency) channels at 5 MHz intervals in the ISM band (industrial, scientific, medical) around 2.4 GHz, which is usable almost worldwide. For raw data rates of 250 kbit/s a symbol rate of 62.5 ksymbol/s and a band spread (spreading) with a chip rate of fC=2 Mchip/s and offset QPSK modulation (quaternary phase shift keying) is provided in these channels.
On the receive side, the bandpass radio signal transmitted in the ISM band must first be converted (i.e., transformed) into a digital baseband signal. While other circuit units of the receiving unit need not be activated until after successful synchronization, the device for converting the bandpass signal into the baseband must already be active during what is known as the listening phase (RX listen mode) for the preamble sequence. For this reason, energy consumption of this device is extremely important to the energy consumption of the transmitting/receiving device as a whole.
Known devices for converting a bandpass signal into a digital baseband signal require two analog-to-digital converters to digitize the real and imaginary components of the complex-valued signal that has been shifted into the baseband or an intermediate frequency range. This has the disadvantages of a high implementation cost and increased energy consumption during operation. Moreover, arithmetic units such as filters, etc., for carrying out discrete-time arithmetic operations such as addition and/or multiplication, are often included as well, which is disadvantageous with regard to the implementation cost of the transmitting/receiving device in question and also with regard to the energy consumption during operation.